Biologists have identified four processes of evolution, each one leading to changes in the frequency of alleles in a population over time. Rabbits in a given population have two different alleles for coat color-white and brown. The environment is snow-covered for most of the year, so the white rabbits are better camouflaged than the brown rabbits. Over time, the population of rabbits becomes overwhelmingly white-coated. This is an example of which evolutionary process?